The present invention relates to steering devices and speed reduction devices for use in motor vehicles and the like.
With electric motor-driven power steering devices of the rack-pinion type, the rotation of the motor needs to be transmitted to a rack shaft (movable member) upon speed reduction. With such a conventional power steering device having a d.c. motor incorporated therein, the rotation of the motor is delivered, for example, to a worm reduction gear mechanism for speed reduction and then to the rack shaft by a pinion and a rack.
Another power steering device is also proposed which comprises a housing for supporting a rack shaft, a tubular rotary shaft disposed inside the housing and rotatable by an electric motor, and a ball nut (nut having recirculating balls) disposed inside the housing and rotatably supported, the rack shaft extending through the shaft and the nut. The rotation of the rotary shaft is transmitted through a planetary gear reduction mechanism to the ball nut, the rotation of which is transmitted to the rack shaft via a ball thread mechanism (see Unexamined Japanese Patent Publication SHO 60-154955).
On the other hand, a hydraulic power steering device of the ball thread type is known which comprises a housing having a cylinder portion, a rotary shaft (output shaft) disposed in the cylinder portion and rotatable by the rotation of a steering wheel, and a piston member (movable member) disposed in the cylinder portion and partitioning the interior of this portion into front and rear oil chambers. The rotation of the rotary shaft is transmitted to the piston member through a ball thread mechanism to move the piston member forward or rearward and thereby rotate a pitman shaft.
However, in the case of the conventional motor-driven power steering devices of the rack-pinion type stated, the characteristics of the d.c. motor involve a high-speed rotation and a low torque and therefore require a great reduction ratio, which necessitates a complex reduction mechanism such as those mentioned and exterts a great adverse effect on the performance involving return of the steering wheel. Furthermore, the use of the worm or planetary gear reduction mechanism for reducing the speed of rotation of the motor entails a great transmission loss, so that the motor needs to have an increased capacity. In the case where the ball thread mechanism is used for speed reduction, the reduction ratio is dependent on the lead of thread of the mechanism, whereas it is impossible to greatly change the lead of thread of the mechanism in view of the construction. This presents difficulty in greatly altering the reduction ratio, leading to reduced freedom of design.
The hydraulic power steering device of the ball thread type similarly has the problem of reduced freedom of design since the ball thread mechanism is used for speed reduction.
The same problem as above is also encountered with steering devices other than power steering devices and with other speed reduction devices.